Method of performing a random access process and wireless device using same

ABSTRACT

Provided are a method of performing a random access process and wireless device using same in a wireless communication system. A first random access process and a second random access process are triggered in one subframe, and a wireless device selects one of the first random access process and the second random access process. The wireless device transmits a random access preamble on the selected random access process from the one subframe to a base station.

TECHNICAL FIELD

The present invention relates to wireless communication, and moreparticularly, to a method of performing a random access procedure in awireless communication system, and a wireless device using the method.

BACKGROUND ART

Long term evolution (LTE) based on 3^(rd) generation partnership project(3GPP) technical specification (TS) release 8 is a promisingnext-generation mobile communication standard. Recently, LTE-advanced(LTE-A) based on 3GPP TS release 10 supporting multiple carriers isunder standardization.

Multiple carriers are supported starting from 3GPP LTE-A, and such atechnique is called a carrier aggregation. One carrier corresponds toone cell, and as a result, a user equipment can receive a service from aplurality of serving cells in a multiple-carrier system.

A random access procedure is used to maintain an uplink time alignmentbetween a base station and a user equipment or to deliver a schedulingrequest. In general, the random access procedure includes transmissionof a random access preamble and reception of a random access response.

It has been conventionally considered that the random access procedureis performed only in one cell. However, with the introduction of aplurality of serving cells, there is a need to design a random accessprocedure performed in the plurality of serving cells.

DISCLOSURE OF THE INVENTION

The present invention provides a method of performing a random accesswhen a plurality of random access procedures are triggeredsimultaneously, and a wireless device using the method.

In one aspect, there is provided a method of performing a random accessprocedure in a wireless communication system. The method may comprise:selecting one of a first random access procedure and a second randomaccess procedure if the first random access procedure and the secondrandom access procedure are triggered in a single subframe; andtransmitting to a base station a random access preamble for the selectedrandom access procedure in the single subframe.

The first random access procedure may be triggered by an order of thebase station, and the second random access procedure may be triggered bya medium access control (MAC) layer.

The first random access procedure and the second random access proceduremay be triggered in different serving cells

In other aspect, there is provided a wireless device for performing arandom access procedure in a wireless communication system. The wirelessdevice may comprise: a radio frequency (RF) unit for transmitting andreceiving a radio signal; and a processor operatively coupled to the RFunit, wherein the processor is configured to: select one of a firstrandom access procedure and a second random access procedure if thefirst random access procedure and the second random access procedure aretriggered in a single subframe; and transmit to a base station a randomaccess preamble for the selected random access procedure in the singlesubframe.

A method of selectively performing a random access procedure is proposedwhen a plurality of random access procedures are triggered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a downlink radio frame in 3^(rd) generationpartnership project (3GPP) long term evolution-advanced (LTE-A).

FIG. 2 shows an example of monitoring a physical downlink controlchannel (PDCCH).

FIG. 3 shows an example of multiple carriers.

FIG. 4 shows an example of cross-component carrier (CC) scheduling.

FIG. 5 is a flowchart showing a random access procedure in 3GPPLTE/LTE-A.

FIG. 6 shows an example of a random access response.

FIG. 7 shows an example of triggering a plurality of random accessprocedures.

FIG. 8 is a flowchart showing a method of performing a random accessaccording to an embodiment of the present invention.

FIG. 9 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

MODES FOR CARRYING OUT THE PREFERRED EMBODIMENTS

A wireless device may be fixed or mobile, and may be referred to asanother terminology, such as a user equipment (UE), a mobile station(MS), a mobile terminal (MT), a user terminal (UT), a subscriber station(SS), a wireless device, a personal digital assistant (PDA), a wirelessmodem, a handheld device, etc. The wireless device may also be a devicesupporting only data communication such as a machine-type communication(MTC) device.

A base station (BS) is generally a fixed station that communicates withthe wireless device, and may be referred to as another terminology, suchas an evolved-NodeB (eNB), a base transceiver system (BTS), an accesspoint, etc.

Hereinafter, it is described that the present invention is appliedaccording to a 3^(rd) generation partnership project (3GPP) long termevolution (LTE) based on 3GPP technical specification (TS) release 8 or3GPP LTE-advanced (LTE-A) based on 3GPP TS release 10. However, this isfor exemplary purposes only, and thus the present invention is alsoapplicable to various wireless communication networks.

FIG. 1 shows a structure of a downlink radio frame in 3GPP LTE-A. Thesection 6 of 3GPP TS 36.211 V10.2.0 (2011-06) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 10)” may be incorporated herein by reference.

A radio frame includes 10 subframes indexed with 0 to 9. One subframeincludes 2 consecutive slots. A time required for transmitting onesubframe is defined as a transmission time interval (TTI). For example,one subframe may have a length of 1 millisecond (ms), and one slot mayhave a length of 0.5 ms.

One slot may include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlink(DL), the OFDM symbol is only for expressing one symbol period in thetime domain, and there is no limitation in a multiple access scheme orterminologies. For example, the OFDM symbol may also be referred to asanother terminology such as a single carrier frequency division multipleaccess (SC-FDMA) symbol, a symbol period, etc.

Although it is described that one slot includes 7 OFDM symbols forexample, the number of OFDM symbols included in one slot may varydepending on a length of a cyclic prefix (CP). According to 3GPP TS36.211 V10.2.0, in case of a normal CP, one slot includes 7 OFDMsymbols, and in case of an extended CP, one slot includes 6 OFDMsymbols.

A resource block (RB) is a resource allocation unit, and includes aplurality of subcarriers in one slot. For example, if one slot includes7 OFDM symbols in a time domain and the RB includes 12 subcarriers in afrequency domain, one RB can include 7×12 resource elements (REs).

A DL subframe is divided into a control region and a data region in thetime domain. The control region includes up to first four OFDM symbolsof a first slot in the subframe. However, the number of OFDM symbolsincluded in the control region may vary. A physical downlink controlchannel (PDCCH) and other control channels are allocated to the controlregion, and a physical downlink shared channel (PDSCH) is allocated tothe data region.

A UL subframe may be divided into a control region and a data region.The control region is a region to which a physical uplink controlchannel (PUCCH) carrying UL control information is allocated. The dataregion is a region to which a physical uplink shared channel (PUSCH)carrying user data is allocated.

Now, a DL control channel is described.

As disclosed in 3GPP TS 36.211 V10.2.0, examples of a physical controlchannel in 3GPP LTE/LTE-A include a physical downlink control channel(PDCCH), a physical control format indicator channel (PCFICH), and aphysical hybrid-ARQ indicator channel (PHICH). In addition, a controlsignal transmitted in a physical layer may be a primary synchronizationsignal (PSS), a secondary synchronization signal (SSS), and a randomaccess preamble.

The PSS is transmitted in last OFDM symbols of a 1^(st) slot (or a1^(st) subframe (i.e., a subframe with an index 0) and an 11^(th) slot(or a 6^(th) subframe (i.e., a subframe with an index 5). The PSS isused to attain OFDM symbol synchronization or slot synchronization, andis in association with a physical cell identify (ID). A primarysynchronization code (PSC) is a sequence used for the PSS. There arethree PSCs in the 3GPP LTE. One of the three PSCs is transmitted usingthe PSS according to the cell ID. The same PSC is used for each of thelast OFDM symbols of the 1^(st) slot and the 11^(th) slot.

The SSS includes a 1^(st) SSS and a 2^(nd) SSS. The 1^(st) SSS and the2^(nd) SSS are transmitted in an OFDM symbol adjacent to an OFDM symbolin which the PSS is transmitted. The SSS is used to attain framesynchronization. The SSS is used to attain a cell ID together with thePSS. The 1^(st) SSS and the 2^(nd) SSS use different secondarysynchronization codes (SSCs). If the 1^(st) SSS and the 2^(nd) SSS eachinclude 31 subcarriers, sequences of two SSCs with a length of 31 arerespectively used for the 1^(st) SSS and the 2^(nd) SSS.

The PCFICH transmitted in a first OFDM symbol of the subframe carries acontrol format indicator (CFI) regarding the number of OFDM symbols(i.e., a size of the control region) used for transmission of controlchannels in the subframe. A wireless device first receives the CFI onthe PCFICH, and thereafter monitors the PDCCH.

Unlike the PDCCH, the PCFICH does not use blind decoding, and istransmitted by using a fixed PCFICH resource of the subframe.

The PHICH carries a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal for an uplink hybridautomatic repeat request (HARQ).

The ACK/NACK signal for uplink (UL) data on a PUSCH transmitted by thewireless device is transmitted on the PHICH.

A physical broadcast channel (PBCH) is transmitted in first four OFDMsymbols in a second slot of a first subframe of a radio frame. The PBCHcarries system information necessary for communication between thewireless device and a BS. The system information transmitted through thePBCH is referred to as a master information block (MIB). In comparisonthereto, system information transmitted on the PDCCH is referred to as asystem information block (SIB).

Control information transmitted through the PDCCH is referred to asdownlink control information (DCI). The DCI may include resourceallocation of the PDSCH (this is referred to as a downlink (DL) grant),resource allocation of a PUSCH (this is referred to as an uplink (UL)grant), a set of transmit power control commands for individual UEs inany UE group, and/or activation of a voice over Internet protocol(VoIP).

The 3GPP LTE/LTE-A uses blind decoding for PDCCH detection. The blinddecoding is a scheme in which a desired identifier is de-masked from acyclic redundancy check (CRC) of a received PDCCH (referred to as acandidate PDCCH) to determine whether the PDCCH is its own controlchannel by performing CRC error checking.

The BS determines a PDCCH format according to DCI to be transmitted tothe UE, attaches a CRC to the DCI, and masks a unique identifier(referred to as a radio network temporary identifier (RNTI)) to the CRCaccording to an owner or usage of the PDCCH.

A control region in a subframe includes a plurality of control channelelements (CCEs). The CCE is a logical allocation unit used to providethe PDCCH with a coding rate depending on a radio channel state, andcorresponds to a plurality of resource element groups (REGs). The REGincludes a plurality of resource elements. According to an associationrelation of the number of CCEs and the coding rate provided by the CCEs,a PDCCH format and the number of bits of an available PDCCH aredetermined.

One REG includes 4 REs. One CCE includes 9 REGs. The number of CCEs usedto configure one PDCCH may be selected from a set {1, 2, 4, 8}. Eachelement of the set {1, 2, 4, 8} is referred to as a CCE aggregationlevel.

The BS determines the number of CCEs used in transmission of the PDCCHaccording to a channel state. For example, a wireless device having agood DL channel state may use one CCE in PDCCH transmission. A wirelessdevice having a poor DL channel state may use 8 CCEs in PDCCHtransmission.

A control channel consisting of one or more CCEs performs interleavingon an REG basis, and is mapped to a physical resource after performingcyclic shift based on a cell identifier (ID).

FIG. 2 shows an example of monitoring a PDCCH. The section 9 of 3GPP TS36.213 V10.2.0 (2011-06) may be incorporated herein by reference.

The 3GPP LTE uses blind decoding for PDCCH detection. The blind decodingis a scheme in which a desired identifier is de-masked from a CRC of areceived PDCCH (referred to as a PDCCH candidate) to determine whetherthe PDCCH is its own control channel by performing CRC error checking. Awireless device cannot know about a specific position in a controlregion in which its PDCCH is transmitted and about a specific CCEaggregation or DCI format used for PDCCH transmission.

A plurality of PDCCHs may be transmitted in one subframe. The wirelessdevice monitors the plurality of PDCCHs in every subframe. Monitoring isan operation of attempting PDCCH decoding by the wireless deviceaccording to a format of the monitored PDCCH.

The 3GPP LTE uses a search space to reduce a load of blind decoding. Thesearch space may also be called a monitoring set of a CCE for the PDCCH.The wireless device monitors the PDCCH in the search space.

The search space is classified into a common search space and aUE-specific search space. The common search space is a space forsearching for a PDCCH having common control information and consists of16 CCEs indexed with 0 to 15. The common search space supports a PDCCHhaving a CCE aggregation level of {4, 8}. However, a PDCCH (e.g., DCIformats 0, 1A) for carrying UE-specific information may also betransmitted in the common search space. The UE-specific search spacesupports a PDCCH having a CCE aggregation level of {1, 2, 4, 8}.

Table 1 shows the number of PDCCH candidates monitored by the wirelessdevice.

TABLE 1 Number Search Space Aggregation Size of PDCCH Type level L [InCCEs] candidates DCI formats UE-specific 1 6 6 0, 1, 1A, 2 12 6 1B, 1D,2, 2A 4 8 2 8 16 2 Common 4 16 4 0, 1A, 1C, 8 16 2 3/3A

A size of the search space is determined by Table 1 above, and a startpoint of the search space is defined differently in the common searchspace and the UE-specific search space. Although a start point of thecommon search space is fixed irrespective of a subframe, a start pointof the UE-specific search space may vary in every subframe according toa UE identifier (e.g., C-RNTI), a CCE aggregation level, and/or a slotnumber in a radio frame. If the start point of the UE-specific searchspace exists in the common search space, the UE-specific search spaceand the common search space may overlap with each other.

In a CCE aggregation level LE {1,2,3,4}, a search space S^((L)) _(k) isdefined as a set of PDCCH candidates. A CCE corresponding to a PDCCHcandidate m of the search space S^((L)) _(k) is given by Equation 1below.

L·{(Y _(k) +m′)mod └N _(CCE,k) /L┘}+i  [Equation 1]

Herein, i=0, 1, . . . , L−1, m=0, . . . , M^((L))−1, and N_(CCE,k)denotes the total number of CCEs that can be used for PDCCH transmissionin a control region of a subframe k. The control region includes a setof CCEs numbered from 0 to N_(CCE,k)−1. M^((L)) denotes the number ofPDCCH candidates in a CCE aggregation level L of a given search space.

If a carrier indicator field (CIF) is set to the wireless device,m′=m+M^((L))n_(cif). Herein, n_(cif) is a value of the CIF. If the CIFis not set to the wireless device, m′=m.

In a common search space, Y_(k) is set to 0 with respect to twoaggregation levels L=4 and L=8.

In a UE-specific search space of the aggregation level L, a variableY_(k) is defined by Equation 2 below.

Y _(k)=(A·Y _(k-1))mod D  [Equation 2]

Herein, Y⁻¹=n_(RNTI)≠0, A=39827, D=65537, k=floor(n_(s)/2), and n_(s)denotes a slot number in a radio frame.

In 3GPP LTE/LTE-A, transmission of a DL transport block is performed ina pair of the PDCCH and the PDSCH. Transmission of a UL transport blockis performed in a pair of the PDCCH and the PUSCH. For example, thewireless device receives the DL transport block on a PDSCH indicated bythe PDCCH. The wireless device receives a DL resource assignment on thePDCCH by monitoring the PDCCH in a DL subframe. The wireless devicereceives the DL transport block on a PDSCH indicated by the DL resourceassignment.

Now, a multiple carrier system is described.

A 3GPP LTE system supports a case in which a DL bandwidth and a ULbandwidth are differently configured under the premise that onecomponent carrier (CC) is used. The 3GPP LTE system supports up to 20MHz, and the UL bandwidth and the DL bandwidth may be different fromeach other. However, only one CC is supported in each of UL and DLcases.

Spectrum aggregation (also referred to as bandwidth aggregation orcarrier aggregation) supports a plurality of CCs. For example, if 5 CCsare assigned as a granularity of a carrier unit having a bandwidth of 20MHz, a bandwidth of up to 100 MHz can be supported.

One DL CC or a pair of a UL CC and a DL CC may be mapped to one cell.Therefore, when a wireless device communicates with a BS through aplurality of DL CCs, it can be said that the wireless device receives aservice from a plurality of serving cells.

FIG. 3 shows an example of multiple carriers.

Although three DL CCs and three UL CCs are shown herein, the number ofDL CCs and the number of UL CCs are not limited thereto. A PDCCH and aPDSCH are independently transmitted in each DL CC. A PUCCH and a PUSCHare independently transmitted in each UL CC. Since three DL CC-UL CCpairs are defined, it can be said that the wireless device receives aservice from three serving cells.

The wireless device may monitor the PDCCH in a plurality of DL CCs, andmay receive a DL transport block simultaneously via the plurality of DLCCs. The wireless device may transmit a plurality of UL transport blockssimultaneously via a plurality of UL CCs.

It is assumed that a pair of a DL CC #1 and a UL CC #1 is a 1^(st)serving cell, a pair of a DL CC #2 and a UL CC #2 is a 2^(nd) servingcell, and a DL CC #3 is a 3^(rd) serving cell. Each serving cell may beidentified by using a cell index (CI). The CI may be cell-specific orUE-specific. Herein, CI=0, 1, 2 are assigned to the 1^(st) to 3^(rd)serving cells for example.

The serving cell may be classified into a primary cell and a secondarycell. The primary cell operates at a primary frequency, and is a celldesignated as the primary cell when the wireless device performs aninitial network entry process or starts a network re-entry process orperforms a handover process. The primary cell is also called a referencecell. The secondary cell operates at a secondary frequency. Thesecondary cell may be configured after a radio resource control (RRC)connection is established, and may be used to provide an additionalradio resource. At least one primary cell is configured always. Thesecondary cell may be added/modified/released by using higher-layersignaling (e.g., RRC messages).

The CI of the primary cell may be fixed. For example, a lowest CI may bedesignated as a CI of the primary cell. It is assumed hereinafter thatthe CI of the primary cell is 0 and a CI of the secondary cell isallocated sequentially starting from 1.

The UE may monitor a PDCCH through a plurality of serving cells.However, even if there are N serving cells, the BS may be configured tomonitor the PDCCH for M (M≦N) serving cells. In addition, the BS may beconfigured to preferentially monitor the PDCCH for L (L≦M≦N) servingcells.

The multiple carrier system can use two types of scheduling.

First, according to per-CC scheduling, PDSCH scheduling is performedonly in each serving cell. A PDSCH of a primary cell is scheduled in aPDCCH of the primary cell, and a PDSCH of a secondary cell is scheduledin a PDCCH of the secondary cell. Accordingly, a PDCCH-PDSCH structureof the conventional 3GPP LTE may be directly used.

Second, according to cross-CC scheduling, a PDCCH of each serving cellmay schedule not only its PDSCH but also a PDSCH of another servingcell.

A serving cell in which the PDCCH is transmitted is called a schedulingcell, and a serving cell in which the PDSCH to be scheduled istransmitted through the PDCCH of the scheduling cell is called ascheduled cell. The scheduling cell may also be called a scheduling CC,and the scheduled CC may also be called a scheduled CC. According to theper-CC scheduling, the scheduling cell and the scheduled cell areidentical. According to the cross-CC scheduling, the scheduling cell andthe scheduled cell may be identical or different.

For the cross-CC scheduling, a carrier indicator field (CIF) isintroduced in DCI. The CIF includes a CI of a cell having a PDSCH to bescheduled. It can be said that the CIF indicates a CI of a scheduledcell. According to the per-CC scheduling, the CIF is not included in DCIof a PDCCH. According to the cross-CC scheduling, the CIF is included inthe DCI of the PDCCH.

The BS may configure the per-CC scheduling or the cross-CC scheduling ina cell-specific or UE-specific manner. For example, the BS may configurethe cross-CC scheduling to a specific UE by using a higher layer messagesuch as an RRC message.

Even if there are a plurality of serving cells, the BS may monitor thePDCCH only in a specific serving cell to decrease a load of blinddecoding. A cell activated to monitor the PDCCH is called an activatedcell (or a monitoring cell).

FIG. 4 shows an example of cross-CC scheduling.

A UE detects a PDCCH 510. Then, on the basis of DCI on the PDCCH 510,the UE receives a DL transport block on a PDSCH 530. Even if thecross-CC scheduling is configured, a PDCCH-PDSCH pair in the same cellmay be used.

The UE detects a PDCCH 520. Assume that a CIF included in DCI on thePDCCH 520 indicates a second serving cell. The UE receives a DLtransport block on a PDSCH 540 of the second serving cell.

Now, a random access procedure is described.

FIG. 5 is a flowchart showing a random access procedure in 3GPPLTE/LTE-A.

A wireless device receives a root index and a physical random accesschannel (PRACH) configuration index from a BS. Each cell has 64candidate random access preambles defined by a Zadoff-Chu (ZC) sequence.The root index is a logical index for generating the 64 candidate randomaccess preambles by the wireless device.

The random access preamble is limited to a specific time and frequencyresource for each cell. The PRACH configuration index indicates aspecific subframe and preamble format capable of transmitting the randomaccess preamble.

Table 2 below shows an example of the random access configurationdisclosed in the section 5.7 of 3GPP TS 36.211 V8.7.0 (2009-05).

TABLE 2 PRACH Preamble System frame Subframe configuration index formatnumber number 0 0 Even 1 1 0 Even 4 2 0 Even 7 3 0 Any 1 4 0 Any 4 5 0Any 7 6 0 Any 1, 6

The wireless device transmits a randomly selected random access preambleto the BS (step S110). The wireless device selects one of the 64candidate random access preambles. In addition, the wireless deviceselects a corresponding subframe by using the PRACH configuration index.The wireless device transmits the selected random access preamble in theselected subframe.

Upon receiving the random access preamble, the BS transmits a randomaccess response (RAR) to the wireless device (step S120). The RAR isdetected in two steps. First, the wireless device detects a PDCCH maskedwith a random access-RNTI (RA-RNTI). The wireless device receives theRAR included in a medium access control (MAC) protocol data unit (PDU)through a PDCCH indicated by the detected PDCCH.

FIG. 6 shows an example of a random access response (RAR).

The RAR may include a timing advance command (TAC), a UL grant, and atemporary C-RNTI.

The TAC is information indicating a time alignment value sent by a BS toa wireless device to maintain a UL time alignment. The wireless deviceupdates UL transmission timing by using the time alignment value. Whenthe wireless device updates the time alignment, a time alignment timerstarts or restarts. The wireless device may perform UL transmission onlywhen the time alignment timer is running.

The UL grant is a UL resource used in transmission of a schedulingmessage described below.

Referring back to FIG. 5, the wireless device transmits a scheduledmessage to the BS according to a UL grant included in the RAR (stepS130).

Hereinafter, the random access preamble, the RAR, and the scheduledmessage may also be called messages M1, M2, and M3, respectively.

The random access procedure may be triggered by at least one of thefollowings.

(1) Triggering by a MAC layer: The MAC layer of the wireless device maytrigger the random access procedure to request UL scheduling.Hereinafter, the random access procedure triggered by the MAC layer iscalled a MAC-random access procedure.

(2) Triggering by a PDCCH order: A BS may instruct the wireless deviceto start the random access procedure due to a cause of UL time alignmentor the like. The random access procedure is triggered when a specificfield masked with a C-RNTI and having a DCI format 1A is set to aspecific value. The PDCCH order may enable per-CC scheduling or cross-CCscheduling. Hereinafter, a random access procedure triggered by an orderof the BS is called a PDCCH-random access procedure. The PDCCH-randomaccess procedure may transmit a randomly selected random accesspreamble, or may transmit a dedicated random access preamble.

In 3GPP LTE/LTE-A, there is a restriction in that the random accessprocedure is performed only through a primary cell even if the wirelessdevice has a plurality of serving cells. Both of the MAC-random accessprocedure and the PDCCH-random access procedure are performed only inthe primary cell.

However, if frequency bands of the plurality of serving cells areseparated from one another, a primary cell and a second cell may havedifferent frequency features, and thus a random access procedure for ULtime alignment may also need to be performed in the secondary cell.

In addition, although it is assumed in 3GPP LTE/LTE-A that the pluralityof serving cells are managed by one BS, it is also possible to considera case where the plurality of serving cells are managed by a pluralityof BSs. This implies that the wireless device has a plurality of MAClayers. When the plurality of MAC layers operate independently, theremay be a case where a plurality of random access procedures aresimultaneously triggered by the plurality of MAC layers in the same timepoint (i.e., the same subframe).

FIG. 7 shows an example of triggering a plurality of random accessprocedures.

In a subframe n, a PDCCH-random access procedure of a secondary cell istriggered in a primary cell by a PDCCH order 710. This is for a BS toacquire UL timing information of the secondary cell. The PDCCH-randomaccess procedure starts in a first subframe n+k (k>=6). The subframe n+kis a UL subframe which satisfies a PRACH configuration. It is assumedherein that a first random access preamble 720 for the PDCCH-randomaccess procedure is transmitted in a subframe n+8.

Meanwhile, in addition to the PDCCH-random access procedure, it is alsoassumed that a second random access preamble 730 for a MAC-random accessprocedure is transmitted for a scheduling request in the subframe n+8.

A plurality of serving cells and/or a plurality of MAC layers may leadto various situations in which transmission of a plurality of randomaccess preambles starts in the same subframe.

The present invention proposes to perform one selected random accessprocedure even if the plurality of random access procedures aretriggered in the same subframe as described above. In doing so,simultaneous transmission of the plurality of random access preamblescan be avoided. This is because excessive UL transmit power is consumedto transmit the plurality of random access preambles, and implementationof the wireless device and the network may become complicated.

FIG. 8 is a flowchart showing a method of performing a random accessaccording to an embodiment of the present invention.

In step S810, a wireless device selects one of a plurality of randomaccess procedures triggered simultaneously. For example, a first randomaccess procedure and a second random access procedure may be triggeredin one subframe. Although it is considered hereinafter that two randomaccess procedures are simultaneously triggered, the number of randomaccess procedures to be triggered is not limited thereto.

The plurality of random access procedures to be triggered may beclassified according to a triggering cause, a dedicated random accesspreamble assignment, a serving cell to be triggered, and a combinationof them.

The first and second random access procedure may have differenttriggering causes. For example, the first random access procedure may bea PDCCH-random access procedure triggered by the aforementioned BSorder, and the second random access procedure may be a MAC-random accessprocedure triggered by a MAC layer of the wireless device.

The first random access procedure may be a non-contention based randomaccess procedure which uses a pre-assigned dedicated random accesspreamble, and the second random access procedure may be a contentionbased random access procedure which uses a randomly selected randomaccess preamble. The PDCCH-random access procedure may be thenon-contention based random access procedure or the contention basedrandom access procedure. In addition, the MAC-random access proceduremay be the non-contention based random access procedure or thecontention based random access procedure.

The first and second random access procedure may be triggered indifferent serving cells. The serving cell to be triggered may include atleast any one of a serving cell for transmitting a random accesspreamble and a serving cell for receiving a random access response. Forexample, the first random access procedure may be triggered in thesecondary cell, and the second random access procedure may be triggeredin the primary cell.

In step S820, the wireless device performs the selected random accessprocedure. Assume that the first random access procedure is selectedbetween the first random access procedure and the second random accessprocedure. The wireless device may transmit a random access preamble forthe first random access procedure to the BS in the aforementioned singlesubframe.

Now, a criterion of selecting one of a plurality of random accessprocedures triggered simultaneously is described.

For clarity, it is assumed that two random access procedures aretriggered simultaneously. Herein, a first random access procedure is aPDCCH-random access procedure, and a second random access procedure is aMAC-random access procedure.

However, this is for exemplary purposes only, and thus there is norestriction on the number of random access procedures to be triggered ora triggering cause.

In a first embodiment, the MAC-random access procedure may be discarded(or stopped), and only the PDCCH-random access procedure may beperformed.

Since a BS intends to acquire UL timing information of a correspondingcell through a PDCCH order, the PDCCH-random access procedure isassigned a higher priority than the MAC-random access procedure. This isbecause, even if the wireless device give up to perform the MAC-randomaccess procedure for a scheduling request, a buffer status report (BSR)can be transmitted through scheduled PUSCH transmission.

In a second embodiment, the MAC-random access procedure may be delayed,and the PDCCH-random access procedure may be first performed.

Assume that a PDCCH order is received in a subframe n, and a randomaccess preamble for the PDCCH-random access procedure is transmitted ina subframe n+k (k>=6). The subframe n+k is a first subframe whichsatisfies a random access configuration. In this case, even if theMAC-random access procedure is triggered in the subframe n+k, a randomaccess preamble for the MAC-random access procedure is not transmitted.The random access preamble for the MAC-random access procedure may betransmitted in a first subframe which satisfies the random accessconfiguration after the subframe n+k.

Alternatively, the random access preamble for the MAC-random accessprocedure may be transmitted in the first subframe which satisfies therandom access configuration after the PDCCH-random access procedure iscomplete. Herein, the ‘after the PDCCH-random access procedure iscomplete’ may imply ‘after an M2 message is received’ or ‘after an M3message is transmitted’. This is to avoid unnecessary overlapping of therandom access procedures.

In doing so, there may be an advantage when the plurality of randomaccess procedures are triggered in a plurality of MAC layers. This isbecause delaying may be more effective in terms of buffer managementthan discarding a random access procedure having a low priority.

In a third embodiment, the PDCCH-random access procedure may bediscarded (or stopped), and only the MAC-random access procedure may beperformed.

The MAC-random access procedure triggered by the MAC layer of thewireless device is necessary to perform direct data communication.Therefore, an increase in a latency for a case of performing datacommunication may have a greater effect to a user than an increase in alatency for a case of acquiring UL timing. Accordingly, the MAC-randomaccess procedure may be assigned a higher priority than the PCCH-randomaccess procedure.

If the random access preamble for the PDCCH-random access procedure isnot received in a corresponding subframe, the BS may transmit a newPDCCH order to the wireless device.

In a fourth embodiment, the PDCCH-random access procedure may bedelayed, and the MAC-random access procedure may be first performed.

Assume that a PDCCH order is received in a subframe n, and thePDCCH-random access procedure is triggered in a subframe n+k (k>=6). Thesubframe n+k is a first subframe which satisfies a random accessconfiguration. In this case, even if the PDCCH-random access procedureis triggered in the subframe n+k, if the MAC-random access procedure istriggered, a random access preamble for the MAC-random access procedureis transmitted in the subframe n+k. Subsequently, the wireless devicemay transmit a random access preamble for the PDCCH-random accessprocedure in a first subframe which satisfies the random accessconfiguration after the subframe n+k.

Alternatively, the random access preamble for the PDCCH-random accessprocedure may be transmitted in the first subframe which satisfies therandom access configuration after the MAC-random access procedure iscomplete. Herein, the ‘after the MAC-random access procedure iscomplete’ may imply ‘after an M2 message is received’ or ‘after an M3message is transmitted’. This is to avoid unnecessary overlapping of therandom access procedures.

If the random access preamble for the PDCCH-random access procedure is adedicated random access preamble, the BS may assign the dedicated randomaccess preamble by considering the delay of the PDCCH-random accessprocedure.

In the aforementioned embodiment, the ‘simultaneous’ triggering of theplurality of random access procedures implies that a transmission timeof the plurality of random access preambles overlaps partially orentirely.

When it is said that the PDCCH-random access procedure and theMAC-random access procedure are triggered simultaneously, it may includea case where the MAC-random access procedure is triggered before acorresponding random access preamble is received after a PDCCH order isreceived. For example, assume that the PDCCH order is received in asubframe n, and a PDCCH-random access procedure is triggered in asubframe n+k (k>=6). When the MAC-random access procedure is triggeredin a subframe n+4, one of the aforementioned first to fourth embodimentsmay be applied.

The PDCCH-random access procedure may include a random access procedurewhich is triggered by using various mechanisms as well as by the PDCCHorder. The MAC-random access procedure may include a random accessprocedure triggered by the wireless device autonomously as well as bythe MAC layer.

The PDCCH-random access procedure and the MAC-random access proceduremay be related to a serving cell to be triggered. For example, thePDCCH-random access procedure may include a random access proceduretriggered by the BS in a secondary cell. The MAC-random access proceduremay include a random access procedure triggered by the wireless devicein a primary cell.

The PDCCH-random access procedure may include a non-contention basedrandom access procedure, and the MAC-random access procedure may includea contention based random access procedure.

FIG. 9 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

A BS 50 includes a processor 51, a memory 52, and a radio frequency (RF)unit 53. The memory 52 is coupled to the processor 51, and stores avariety of information for driving the processor 51. The RF unit 53 iscoupled to the processor 51, and transmits and/or receives a radiosignal. The processor 51 implements the proposed functions, procedures,and/or methods. In the aforementioned embodiment, an operation of the BSmay be implemented by the processor 51.

A wireless device 60 includes a processor 61, a memory 62, and an RFunit 63. The memory 62 is coupled to the processor 61, and stores avariety of information for driving the processor 61. The RF unit 63 iscoupled to the processor 61, and transmits and/or receives a radiosignal. The processor 61 implements the proposed functions, procedures,and/or methods. In the aforementioned embodiment, an operation of thewireless device may be implemented by the processor 61.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The RF unit may include a base-bandcircuit for processing a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory and may be performed by the processor. The memory may be locatedinside or outside the processor, and may be coupled to the processor byusing various well-known means.

Although the aforementioned exemplary system has been described on thebasis of a flowchart in which steps or blocks are listed in sequence,the steps of the present invention are not limited to a certain order.Therefore, a certain step may be performed in a different step or in adifferent order or concurrently with respect to that described above.Further, it will be understood by those ordinary skilled in the art thatthe steps of the flowcharts are not exclusive. Rather, another step maybe included therein or one or more steps may be deleted within the scopeof the present invention.

1. A method of performing a random access procedure in a wirelesscommunication system, the method comprising: selecting one of a firstrandom access procedure and a second random access procedure if thefirst random access procedure and the second random access procedure aretriggered in a single subframe; and transmitting to a base station arandom access preamble for the selected random access procedure in thesingle subframe.
 2. The method of claim 1, wherein the first randomaccess procedure is triggered by an order of the base station, and thesecond random access procedure is triggered by a medium access control(MAC) layer.
 3. The method of claim 2, wherein the first random accessprocedure and the second random access procedure are triggered indifferent serving cells.
 4. The method of claim 3, wherein the selectedrandom access procedure is the first random access procedure.
 5. Themethod of claim 4, further comprising, after transmitting the randomaccess preamble for the selected random access procedure to the basestation, transmitting a random access preamble for the second randomaccess procedure to the base station.
 6. The method of claim 4, whereinthe performing of the second random access procedure is discarded. 7.The method of claim 2, wherein the first random access procedure istriggered when specific fields in control information regarding acontrol channel and received from the base station are set to a specificvalue.
 8. The method of claim 1, wherein the first random accessprocedure is a non-contention based random access procedure, and thesecond random access procedure is a contention based random accessprocedure.
 9. The method of claim 1, wherein the first random accessprocedure and the second random access procedure are triggered indifferent MAC layers.
 10. A wireless device for performing a randomaccess procedure in a wireless communication system, the wireless devicecomprising: a radio frequency (RF) unit for transmitting and receiving aradio signal; and a processor operatively coupled to the RF unit,wherein the processor is configured for: select one of a first randomaccess procedure and a second random access procedure if the firstrandom access procedure and the second random access procedure aretriggered in a single subframe; and transmit to a base station a randomaccess preamble for the selected random access procedure in the singlesubframe.
 11. The wireless device of claim 10, wherein the first randomaccess procedure is triggered by an order of the base station, and thesecond random access procedure is triggered by a medium access control(MAC) layer.
 12. The wireless device of claim 11, wherein the firstrandom access procedure and the second random access procedure aretriggered in different serving cells.
 13. The wireless device of claim12, wherein the selected random access procedure is the first randomaccess procedure.
 14. The wireless device of claim 10, wherein the firstrandom access procedure is a non-contention based random accessprocedure, and the second random access procedure is a contention basedrandom access procedure.